MOPC 511, a mouse IgA immunoglobulin with phosphorylcholine binding activity, has been studied in terms of the amino acid sequence of both the light and heavy chains. The amino acid sequence of the heavy chain variable region showed seven positions of variability when compared to another heavy chain of a mouse IgA with the same specificity. The light chain, however, showed a much larger degree of variability which indicates that the heavy chain plays a dominant role in the binding of this specific hapten. Human HL-A or mouse H-2 histocompatibility antigens are composed of two subunits: a small subunit of 11,000 daltons or beta-2 microglobulin and the large subunit of approximately 34,000 to 37,000 daltons which contains all the alloantigenic determinants. The beta-2 microglobulin can be purified from different species. It has a large degree of homology with the constant regions (CH2) of immunoglobulin heavy chains. The primary structure of the large subunit is now being carefully examined in order to define any possible homology with immunoglobulin heavy chain and also to define its evolutionary relationship in the immune system.